Silicon carbide (SiC) is an attractive semiconductor material for power devices due to its excellent material properties. The high critical electric field strength makes SiC advantageous for unipolar devices with low on-resistance (Ron). SiC Schottky barrier diodes (SBDs) in the >600V rating are becoming popular. Minimal reverse recovery losses are among the advantages provided by these high voltage (HV) SBDs over conventional silicon PiN diodes. For these HV SBDs, there is no minority stored charge loss; main losses are due to the capacitance charge of the SBD. Other advantages provided by HV SBDs include a high junction temperature rating, and low forward voltage (Vf) and positive temperature coefficient that allows easy paralleling. Thus SiC SBDs are attractive for high frequency applications, which are typically greater than 100 KHz. In these applications, the switching losses dominate. Since total switching loss is proportional to frequency×Esw, where Esw is switching energy loss, a reduction in Esw is attractive. Esw can be reduced by, for example, decreasing the capacitive charge (Qc) and the peak reverse recovery current (Irrmax).
Conventional SiC SBDs use a single epitaxial layer to support the blocking voltage. The doping and thickness of this single epitaxial layer are selected based on the rated breakdown voltage and a best Ron value. Thus, for a selected breakdown voltage, the doping is almost fixed, and capacitance of the SBD is dependent upon the doping concentration. For example, in the conventional reverse bias case, a higher doping concentration means a thinner depletion layer, and results in an increased capacitance of the diode. This conventional design results in high capacitance, especially at low reverse voltages. High capacitance results in relatively high Qc and Irrmax, thereby increasing switching losses. Conventional SiC SBDs include, for example, a 4A/600V SiC SBD (CSD04060) offered by Cree, Inc. of Durham, N.C.
As depicted in FIG. 1, a conventional SiC SBD 100 structure includes an N+ (highly doped) SiC substrate 102, a single N-type epitaxial layer 104 disposed on substrate 102, and a metal Schottky contact 106 disposed on epitaxial layer 104. In FIG. 2, a conventional SBD 200 with two epitaxial layers is depicted having an N+ SiC substrate 202, an N+ SiC epitaxial layer 204 disposed on substrate 202, N-type SiC epitaxial layer 206 disposed on N+ epitaxial layer 204, and a Schottky contact 208 disposed on N-type epitaxial layer 206. In FIGS. 1 & 2, almost all of the blocking voltage is supported by the topmost N epitaxial layer. N+ epitaxial layer 204 positioned below N-type epitaxial layer 206 supports a non-substantial amount of voltage, and it is typically used to prevent the electric field from reaching the substrate.
In these cases (FIGS. 1 & 2), the reverse recovery losses are determined by the doping concentration and the thickness of topmost N-type epitaxial layer 104 (FIG. 1), 206 (FIG. 2). The capacitance and Qc can be lowered by reducing the doping concentration of the top epitaxial layer, or adjusting the doping and thickness of the top epitaxial layer. These modifications can lower Qc and Irrmax, and thus reduce switching losses in the diode and the associated switch. Lighter surface doping also reduces leakage currents. However, such an adjustment to the top epitaxial layer of conventional SiC SBDs increases the on-resistance (Ron) and forward voltage (Vf), as well as creating fragile breakdown (i.e., low Unclamped Inductive Switching (UIS) capability).
Based on the foregoing, a need still exists for an improved SiC SBD structure to reduce capacitance and switching losses while resulting in only small effects on Ron and Vf.